Custard: Coagulating Protein
The process of changing liquid protein into a solid by heating it is called coagulation. Coagulation is a kind of denaturing. For example, egg white coagulates at about 69°C (156°F). it changes from an almost colorless, transparent, fairly thick liquid to a white solid. The protein in an egg yolk also coagulates when heated.
Protein coagulation is one of the main reasons the texture of food changes when it is cooked. Meat and fish become firm, and batters change from liquids to solids. In fact, most baked goods have a "skeleton" of coagulated milk or egg protein that supports them. Custard is a homogeneous mixture of eggs, milk, and sugar that has been heated to coagulate the protein in the eggs and the milk. The next experiment shows how different amounts of heat affect the coagulation of these proteins. Materials & Equipment
Procedure
Observations Which custard has been properly cooked? Which custard has separated and has the most liquid? When eggs first coagulate, the protein is able to trap and hold other liquids, such as the water in the milk and egg whites. If eggs are cooked too long or at too high a temperature, they become tougher and tougher and can no longer hold water. For example, scrambled eggs from which water has separated have been either overcooked or cooked too quickly. Properly made custard is a smooth, shiny, yellow pudding that slices cleanly when you put a spoon into it. There is no trace of water from either the egg or the milk. All of these custards are good to eat. Refrigerate until you are ready to serve them. The overcooked custards should be drained before eating. They can be topped with fruit. |